1. Field of the Invention
The present invention relates generally to host adapters that interface two I/O buses, and more particularly, to dequeuing blocks from queues for host adapters.
2. Description of Related Art
Hardware control blocks, sometimes called sequencer control blocks, or SCSI command blocks (SCBs), are typically used for transferring information between a software host adapter device driver in a host computer system 106 and a host adapter integrated circuit 120 that controls operations of one or more peripheral devices. Those of skill in the art know methods for queuing SCBs.
For example, see copending and commonly assigned U.S. patent application Ser. No. 07/964,532 entitled “Intelligent SCSI Bus Host Adapter Integrated Circuit,” of Craig A. Stuber et al. filed on Oct. 16, 1992, which is incorporated wherein by reference in its entirety. See also, commonly assigned U.S. Pat. No. 5,625,800 entitled “A Sequencer Control Block Array External To A Host Adapter Integrated Circuit” of Michael D. Brayton et al. filed on Jun. 30, 1994; and also U.S. Pat. No. 5,564,023 entitled “Method for Accessing A Sequencer Control Block By A Host Adapter Integrated Circuit” of B. Arlen Young issued on Oct. 8, 1996, each of which is incorporated herein by reference in its entirety.
A prior art method for queuing SCBs for a system 100, which included a processor 105 in a host system 106 and a host adapter device 120 with a sequencer 125, used a one-dimensional linked list SCB queue 160 within a SCB array 155 in random access memory 150. A device driver executing on microprocessor 105 managed host adapter 120 using SCBs, e.g., hardware control blocks, in SCB queue 160. Each SCB in SCB queue 160 had a next queue site field Q_NEXT, sometimes referred to as field Q_NEXT, which was used to store a SCB site pointer that addressed the next SCB storage site in SCB queue 160.
Queue 160 was delimited by head pointer 141 and tail pointer 142. Head pointer 141 was the number of the site in SCB array 155 storing the SCB to be executed next, and tail pointer 142 was the number of the site in SCB array 155 containing the last SCB transferred from host system 106. Next queue site field Q_NEXT in the site addressed by tail pointer 142 had an invalid value that was used to identify the SCB as the last in queue 160 in addition to tail pointer 142.
When queue 160 was empty, head pointer 141 was an invalid value. When a new SCB was transferred from host system 106 to SCB array 155, head pointer 141 and tail pointer 142 were changed to point to the storage site of the new SCB, i.e., were changed to valid values. Field Q_NEXT in the new SCB was set to an invalid value to indicate that the SCB was the last SCB in queue 160.
As each new SCB was transferred from host system 106 to SCB array 155, tail pointer 142 was used to identify the end of queue 160. Field Q_NEXT in the SCB pointed to by tail pointer 142 was changed to point to the newly transferred SCB, and then tail pointer 142 was changed to point to the storage site of the newly transferred SCB.
A valid head pointer 141 indicated that one or more commands were available for transfer to one or more SCSI targets. The next target selected was specified by information within the SCB addressed by head pointer 141. Host adapter 120 removed the SCB at the head of queue 160 for execution and selected the specified target. Head pointer 141 was changed to point to the next SCB in queue 160, i.e., the value in field Q_NEXT of the SCB removed for execution was copied to head pointer 141. When the selection process was complete, the command contained within the SCB was transferred to the target.
SCBs in queue 160 were executed and targets were selected in the order that the SCBs were received from host system 106. This order is shown by the dashed line in FIG. 1. SCBs were not inserted or removed from the middle of queue 160.
Other methods are known for queuing SCSI command blocks for a SCSI target. See for example, U.S. Pat. Nos. 5,938,747 and 6,006,292, each of which is incorporated herein by reference. Independent of the method used to manage SCBs in a queue within the SCB array, SCBs are executed and targets are selected in the order that the SCBs were received from host system 106. SCBs are not inserted into or removed from the middle of the queue within the SCB array.